Lactose intolerance is a condition exhibiting various symptoms such as abdominal pain and diarrhea caused by lactose present in a food product such as a dairy product due to congenital insufficiency of decomposing lactose. Lactose is a disaccharide composed of galactose and glucose. In order to address lactose intolerance, decomposing in advance the lactose contained in milk or the like to galactose and glucose using the lactase enzyme is carried out in the food manufacturing industry.
Lactase solutions that are used for decomposing lactose contained in milk or the like is conventionally produced by culturing a lactase-producing microorganism, extracting the lactase from the inside of the cells, removing culture-derived contaminants, and purifying the lactase, followed by addition of a stabilizer and then filter sterilization.
Patent Literature 1 (JP S60-18394 B) discloses an invention which is directed to a method for producing lactase from a culture of a certain strain of Kluyveromyces lactis. According to this method, after autolysis of the yeast cells, the resulting crude enzyme solution is passed through a DEAE-cellulose column, resulting in a separation into two active fractions (lactases A and B) by elution with a concentration gradient of NaCl. Patent Literature 1 discloses that these two active fractions are little different in enzymatic properties including thermostability, except that they have a slight difference in pH stability, and thus an enzyme preparation can include a mixture of these active fractions.
From results of the genetic analysis of lactase derived from Kluyveromyces lactis, the lactase is a polypeptide consisting of 1,025 amino acids and its molecular weight is presumed to be 117,618 (Non-Patent Literature 1).
It is further described in Patent Literature 1 that the lactase described therein has an optimum temperature of 40 to 50° C. and is inactivated 45% after 10 minute at 50° C. and 100% after 10 minutes at 55° C. at pH 7.0. However, it is not described therein that this enzyme was actually used to decompose the lactose contained in milk. Therefore, Patent Literature 1 describes nothing about the problem of the decrease in enzyme activity when lactase is added to a raw material milk, in particular inactivation of the enzyme when it is subjected to heat load at or above 40° C. in the milk.